A laser nuclear fusion reactor is expected as an apparatus which serves as a next-generation energy supplier. In the laser nuclear fusion reactor, it is necessary to achieve high power of a laser light used therein. Currently, a laser apparatus which can output a laser light of 100 J is under development as a national project, but such a laser apparatus having large output power has not yet been realized.
In a laser amplification apparatus disclosed in Patent Literature 1, a 3-level dopant (Yb) is added, an excitation light is emitted to a side surface of a YAG disk laser, and a seed light is input to a main surface. As the laser apparatus, the apparatuses disclosed in Patent Literatures 2 and 3 are also known.